I noticed there are a few posts about using custom firmware for polycom, like this one that got no responses.
I found a solution that didn’t involve creating a webserver with an all.xml file directing phones to individual files, using commercial endpoint manager. Unfortunately, it seems that when you set up Firmware Slot 2, it still directs the config to sip.ld, which can be too large when you use the combined firmware. The split firmware is much smaller, so it actually works.
First, you need to download the correct version from the polycom software matrix. Look for your phone model, such as VVX 410, and the latest firmware that says “Yes” in that model’s field. Then grab the Split file for that firmware. To determine which .ld file in the resulting zip file you need, on the phone pull up Home, Settings, Status, Platform, Phone. You’re looking for the “Assembly”, which for my VVX 410 is 3111-46162-001. You’ll notice in your firmware zip a bunch of files that end in .sip.ld, and the beginning is the Assembly for your phone.
Rename that file sip.ld and place that file in the custom firmware dialog for your commercial endpoint manager, and select that firmware slot (you need to load 0.00 or 0.01 to set custom firmware for one of the slots) for your phone template, then create your extension in endpoint manager with the MAC address of the phone.
This works because for some reason the APP_FILE_PATH of the config only uses sip.ld and not the filename you uploaded, so this will get the phone to update. The downside for this is it will cause problems if you have other models, so you may want to do this, reprovision the phone so it updates, then remove the custom firmware.